Analysis of Mineral Phases Deformed at High Pressure

Here, we studied a mineralogical sample deformed experimentally at high pressure. The experiment was conducted with a D-DIA multianvil press at the Bayerisches Geoinstitut in order to investigate how deformation textures are affected by phase transformation. A polycrystalline sample of aragonite (CaCO3) was axially compressed to 20% strain at 3 GPa and 800° C and then statically annealed at lower pressure (2 GPa) in the calcite stability field. The goal was to investigate if textures formed during deformation at high pressure in the aragonite field can be inherited in calcite marbles after decompression. Such a scenario is likely to happen when crustal rocks, such as carbonates, are rapidly subducted, deformed at high pressure and then brought back to the surface during exhumation. The textures of the carbonate may then reveal information about the deformation at greater depth in a subduction zone.

The BRUKER EBSD system allows the user to easily identify and successfully distinguish calcite and aragonite which EDS usually cannot. Also note the successful indexing of the crystallographically similar phases calcite and corundum, without the assistance of EDS. These two phases share the same trigonal crystallographic symmetry and therefore produce similar Kikuchi Patterns. Due to the combination of a high resolution detector and a powerful indexing algorithm (indexing of up to 12 Kikuchi bands), a very high hit rate (> 90%) is achieved on the minerals without affecting the analysis speed.

The presence of accessory phases was unknown before the measurement. These phases (pyrite, zoisite) were identified offline using the Advanced Phase ID feature (see Webinar on “Advanced Phase-ID”). It has the advantage to offer, at any time, a correction/completion of the dataset without additional SEM occupation time.

Conclusive remarks

First results from this sample show both intra- and intergrain nucleation of calcite. In neither case the nuclei of calcite have a systematic orientation relationship with their aragonite hosts or neighbours, such that the deformation CPO (crystallographically preferred orientation) of aragonite, a (001) fiber texture, is not inherited by the calcite. Nevertheless calcite shows a weak CPO, characterized by a broad (108)/(018) fiber texture. This CPO may be due to the fact that the stresses during the annealing stage were completely hydrostatic. Likely nucleation and growth are in this case not controlled by an orientation relation to the host (as has been shown in previous studies) but by the overall stress field.